Splicing extrusions usually involves mechanical fasteners and after the parts
are anodized. But when designing connections with anodized parts allowances have to be made for reducing or building up the part thickness depending on anodizing type.
As far as I can tell the max length thing is more a function of demand balanced by practical necessity. Most metal stock tends to not be supplied in lengths greater that about twenty feet which is fine for most people making most things.
The anodizers with the long tanks that I am aware of tend to be located near industry that has a specific need for those long lengths. An example would be a company called Pioneer Metal Finishing in Greenbay, Wisconsin does anodizing for the paper production industry based there, big long paper rollers and whatnot.
Regarding alloy types and temper, confess I don't know all the ins and outs as I haven't needed to learn them but the first four digits are the actual alloy type and the subsequent letters and numbers (the T-6 part or the H-32) are the tempers and in general the aluminum is extruded from its molten state and tempering happens after the fact either deliberately or naturally in the case of the air hardening types.
For what it's worth there is general correlation between the alloy series number and it's physical qualities. For example 5000 series tend to be softer and more ductile than a 6000 series which tend to have higher tensile strength and hardness. Hence people tend to weld hulls with 5000 and extrude masts with 6000. Alloys series range I think from 1,000 out to 7,000 but have never used anything other than 5 or 6.
Then there are the casting alloys which are a whole other ball of wax. I have experience making molds and casting with aluminum but nothing structural so don't really know much about the casting alloys.
One thing that always sort of blew my mind was that when we had leftover aluminum in the crucible we would pour it into ingot molds which were sort of like how a chocolate bar is segmented. We would come back with a sledge to break the ingot into smaller pieces.
Surprise, you could break a 2" thick by 3" wide section of cast aluminum with a surprising little amount of effort. Again, we were making sculptures and not structural components and there was very rapid cooling
of the ingot mold
, but still, I was shocked.
As it is I happen to have an old Isomat rig which has a very high quality clear anodized finish. That is except for the cast aluminum spreader bases which are welded onto the mast before anodizing. This is because of a reaction that occurs involving the cast alloy, the filler rod used in welding, and the extruded alloy which turns the welded filler and cast base black that I learned about the hard way on a job once.
With the correct filler you can avoid having your welds turn black during anodizing when you are welding extrusion or plate, but as far as I know it's a problem with anodizing the cast alloys. Take a look at your neighbors rig once if he has an older Isomat and you will see what I am talking about.
+1 on the earlier comment about cracking that can occur when post forming anodized parts. Due to the relative increase in hardness of the anodized layer compared to the base, anodized parts suffer fatigue related cracking when post formed which can lead to crevice corrosion
where you want it the least, an area that has already been deformed.